The aim of this paper is to develop a step within the justificatory argumentation that bases the juridical-political application of the principle of humanity in a way that makes it fully compatible with the thesis of the deontological distinction between ethics and law. This step belongs to the metaethical level that serves to justify Kant's principles of justice within the original contract. Specifically, my interpretative thesis holds that this level begins with a theory of action by which the description of the agents or persons that participate in the contract is fully compatible with the fact of pluralism.